The invention relates in general to diode-pumped pulsed solid state (DPSS) lasers. The invention relates in particular to DPSS laser including a Q-switch, and wherein a diode-laser for delivering pump-light pulses is driven by a pulsed or switched power-supply and the Q-switch is synchronously activated at the termination of pump-light pulses.
Rapidly pulsed DPSS lasers are increasingly being used for precision, fine machining operations in electronics and related industries. Rapidly pulsed, here, refers to a range of pulse repetition rate between about 1 kilohertz (KHz) and several megahertz (MHz). One example of such a machining operation is trimming of resistors using a pulsed Nd:YAG laser at a wavelength of 1064 nanometers (nm). This operation requires the use of variable cutting speeds. When cutting speed is varied, pulse repetition rate must be varied correspondingly to maintain a constant width of cut. In order for a predetermined relationship between pulse repetition rate and cutting speed to be reliable, the energy-per-pulse must remain constant as the pulse repetition rate is varied.
Another example of a use of rapidly pulsed DPSS lasers is engraving images in plastic cards such as identification cards. A pulsed Nd:YAG laser at a wavelength of 1064 nm is also useful in this operation. Typically an image is engraved by laser machining a series of pits having variable spacings therebetween, the real concentration of pits determining the form of the image. Variable spacing is achieved by varying pulse repetition rate at a constant machining speed. Here again, the operation is most effective if the energy-per-pulse remains constant as pulse repetition rate is varied.
Prior-art rapidly-pulsed DPSS lasers are typically continuously pumped, and pulses are formed by repeatedly opening and closing a Q-switch, located in the laser""s resonant-cavity. In these prior-art lasers, at pulse-repetition rates greater than about 1/xcfx84m m (where xcfx84m is the characteristic lifetime of excited states of the gain medium), energy-per-pulse is inversely dependent on the pulse-repetition rate. For a gain medium with a relatively long characteristic lifetime such as Nd:YLF, this dependence begins at pulse-repetition rates above about 1 KHz. For a gain-medium with a shorter characteristic lifetime, for example, Nd:YAG, the dependence begins above about 4 KHz.
There is a need for a pulsed-laser which provides laser output-pulses having a constant energy-per-pulse independent of the time-interval between the pulses. Preferably, laser output-pulses should have constant energy-per-pulse even if in a series of laser output-pulses the time interval between pulses varies.
A pulsed laser in accordance with the present invention provides output-pulses of constant energy-per-pulse, independent of the interval between pulses, even when intervals between pulses are randomly varying.
In one aspect, a laser in accordance with present invention comprises a laser resonant-cavity or laser resonator having a solid-state gain medium. A source of pump-light is provided for energizing the gain medium. The pump-light source is arranged to provide a series of pump-light pulses for energizing the solid-state gain-medium. Each of the pump-light pulses has the same duration, however, the time-period between pump-light pulses is variable.
A Q-switch is located in the resonant-cavity. The Q-switch is arranged to retard operation of the resonant-cavity until a pump-light pulse is terminated. Termination of a pump-light pulse provides a trigger signal for opening the Q-switch, thereby allowing operation of the resonant-cavity for generating a laser output-pulse. The pump-light source has an essentially constant output throughout each pump-light pulse, whereby each laser output-pulse has about the same energy, independent of the time-period between pulses.
Preferably the pump-light source is further arranged to deliver sufficient pump-light to the gain-medium, between termination of each pump-light pulse and initiation of a subsequent pump-light pulse, that gain in the gain-medium is the same at the initiation of each pump-light pulse independent of the time interval between the pump-light pulses.
In one preferred embodiment, the pump-light source is a diode-laser array driven by a regulated current-supply and a controller. The controller is arranged such that the current-supply delivers a series of current-pulses to the diode-laser array. The diode-laser array responsively generates a series of pump-light pulses for energizing the solid-state gain-medium.
The controller is further arranged such that each of the current-pulses and corresponding pump-light pulses has the same duration, and such that the current-pulses and corresponding pump-light pulses may have a variable time-period therebetween. Termination of a current-pulse provides a trigger-signal for opening the Q-switch, thereby allowing operation of the resonant-cavity for generating a laser output-pulse. The controller is further arranged such that the diode-laser array has an essentially constant output throughout each pump-light pulse, whereby each laser output-pulse has about the same energy, independent of the time-period between laser output-pulses.
In another preferred embodiment the pump-light source is a diode-laser array driven by a regulated current-supply and a controller and provides a continuous pump-light output. A light-modulator is located between the diode-laser array and the solid-state gain-medium in the path of the pump-light output of the diode-laser array. The controller is arranged to operate the light-modulator such that the pump-light output from the diode-laser array is delivered to the gain medium as a series of pump-light pulses having the same duration but having a variable time-period therebetween. A Q-switch is located in the resonator, the Q-switch is arranged to retard operation of the resonator until a pump-light pulse is terminated, the termination of the pump-light pulse proves a trigger-signal for opening the Q-switch, thereby allowing delivery by the resonator of a laser output-pulse. The controller is further arranged such that the diode-laser array has an essentially constant output throughout each pump-light pulse, whereby each laser output-pulse has about the same energy, independent of the time-period between laser output-pulses.